Don't teach so much. Limiting the number of points taught will lead to increased retention. Quality, not quantity.
Make sure your learners are "with you." If the learner isn't attentive, forget it. Move and and return to teaching when the learner is ready. You are wasting your time if they are paying attention.
Be creative in adapting your teaching style when it is busy. You don't have to be at a dry erase board drawing metabolic pathways (sorry Fermin) to be teaching. Simply discussing your thought process outloud is a great way of teaching "on the fly."
Be flexible and remember: the focus should be on the learner (what they get out of it) and not the teacher. Many forget that when they teach in the ED.
Only reference really is the great teaching faculty at the University of Maryland....